A study of the physicochemical and rheological properties for zinc added skim milk : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology, Massey University, Auckland, New Zealand
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Date
2021
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Massey University
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Abstract
Zinc is present in bovine milk in low concentration (0.37 mg per 100g) but in this form is highly
bioavailable. Addition of soluble zinc salts to bovine milk can increase the zinc content.
However, changes in physicochemical and rheological properties in milk after addition of zinc
salts has not been studied extensively. Thus, the aim of this study was to to investigate the
physicochemical and rheological changes in skim milk after the addition of different zinc salts.
The skim milk pH, zinc and calcium distribution were monitored to determine the changes in
physicochemical properties after zinc salts were added. The changes in the skim milk viscosity
and storage modulus were monitored to study the changes in rheological properties. This
research assessed the effect of added zinc salt concentration (0 - 40 mmol L-1), type of zinc
salt added (zinc acetate, zinc sulphate and zinc gluconate), pH, preheat treatment, and
holding temperature (20°C to 80°C).
The addition of all three zinc salts lead to a significant decrease (p<0.05) in skim milk pH, and
the order of final pH in the zinc+skim milk from lowest to highest at the same concentration
level was zinc sulphate < zinc gluconate < zinc acetate. An increase in the serum and sediment
(colloidal phase) zinc concentration was observed after zinc salt was added. As the zinc
acetate concentration increased there was a corresponding decrease in the native calcium
concentration in the collodial phase. When 40 mmol L-1 zinc acetate was added to skim milk,
about 33% of the native calcium present in skim milk was released from the colloidal phase
to the serum.
Rheological measurements using cone and plate geometry at constant strain showed that the
addition of zinc salt could lead to an increase in zinc+skim milk viscosity and with further
increase in the added zinc salt concentration, such as 22.5 mmol L-1 of zinc acetate, 30 mmol
L-1 of zinc sulphate or zinc gluconate, the non-preheated skim milk started to form a gel at
room temperature (20 ± 1°C). The increase in gel strength correlated with the colloidal phase
zinc concentration; as a higher colloidal phase zinc concentration resulted in a stronger gel.
Gelation was also observed for the zinc+skim milk which had its pH adjusted back to 6.73 ± 0.03 after pH drop with the added zinc, but the gel strength was significantly less than when
the pH was not adjusted.
When skim milk samples were preheated, a higher final G’ was achieved at the same
concentration of zinc salt likely due to the participation of denatured whey proteins in the gel
network. The addition of 15 mmol L-1 zinc acetate to preheated skim milk followed by heating
at 80°C then cooling to 20°C formed strong skim milk gels with a final G’ of 104.35 ± 0.94 Pa,
while the non-preheated sample obtained a lower final G’ of 98.82 ± 0.02 Pa. A higher holding
temperature also resulted in higher final G’ of gels. In conclusion, the final gel properties of a
zinc+skim milk sample were influenced by zinc salt concentration added, type of zinc salt
added, pH, preheat treatment and the temperature during gelation. These findings provide
valuable information about the zinc-protein equilibrium in skim milk and offer an alternative
method for texture modification in skim milk.
Description
The following Figures have been removed for copyright reasons: 2.1 (=Berton et al., 2012 Fig 2A); 2.2 (=Dalgleish, 2011 Fig 1); 2.3 (=Horne, 2006 Fig 1); 2.5 (=Horne, 1998 Fig 3); 2.6 (=Ortega-Requena & Rebouillat, 2015 Fig 7); 2.7 (=Gaucheron, 2005 Fig 4); 2.8 (=Ahmad et al., 2009 Fig 9); 2.9 (= Anema, 2009 Fig 1c(i) & c(ii)).